The present invention relates to a method for detection of volatile gases that are specific to the burning material, which are released before a fire in the phase of thermal decomposition. Monitoring these gases makes a pre-alarm and an alarm possible in order to take adequate preventive measures. The invention furthermore describes a novel fire detector which uses a detection of positive and negative ions of volatile gases by separating them after a passage through an electromagnetic field. Thus, it is possible to detect substance-specific thermolysis products in very low concentrations at a very early stage of the development of a fire.
The invention is adapted for use for fire detection, in diverse fields where smoldering fires (very slow temperature rise of the burning material) and thermal decomposition processes must be reckoned with. In the wood-processing industry, the food industry, IT, the field of telecommunication and stockpiling, for instance.
Conventional fire detectors are typically smoke detectors, heat detectors, or flame detectors. They rely on measuring physical values such as temperature, electromagnetic radiation, as well as light scattering by smoke aerosols. In addition to the detection of these classical fire parameters, gases can be detected in an early stage of the thermal decomposition. Smoldering fires which are not detected or detected too late in their formation phase often cause great damage. During the thermal decomposition process in a smoldering fire, gaseous products are released in different concentrations. CO, H2, CH4 and nitrogen oxides, for instance are among these. In the further development of the fire and with an increase of the temperature, the emission of products of a complete combustion such as CO2 and H2O increases. These gases emitted during the formation phase of a fire can be detected at an early stage by using adequate gas sensor systems. Fire gas detectors using known sensors such as electrochemical cells, heat tone gas sensors, semiconductor gas sensors/sensor arrays and infrared absorption gas sensors are known. In addition to low-molecular fire gases such as CO, H2, CH4, NOX, CO2, and H2O, which are formed with a sufficiently high energy supply, substance-specific high-molecular gases are also formed in the thermal decomposition process with a slightly lower energy supply and lower temperatures in the potential burning material, also referred to in short as material. In the following, these thermolysis products are referred to as substance-specific volatile thermolysis products characteristic of the material to be monitored. Examples include, for instance with wood: carboxylic acids, furan-derivatives, aldehydes, ketones and monoaromates. With polyurethane foams: toluene diisocyanate, and polyols.
A disadvantage of the mentioned conventional fire detectors and gas sensors is that they react only at an advanced decomposition stage of the burning material or after the outbreak of the fire. The fire parameters furthermore are not substance-specific. Fire alarms can furthermore be triggered by the influence of identical physical values in the environment. In the following, all measured values which serve for fire detection are referred to as fire parameters.
Keeping the intervention period, which extends from the formation of the fire and the triggering of a fire alarm signal to a fully-fledged firefighting, as short as possible depends particularly on an earliest possible detection of fire parameters.
A method for fire detection, which is supposedly adapted to recognize an increased risk of a fire outbreak based on measuring gases or vapors, is known from DE60005789T2 and WO/0045354. The method disclosed therein is supposed to be more specifically advantageous for the thermal heating and gas emissions of an electric component. This method is based on the known ion mobility spectrometry. It describes the detection of gases which are emitted during heating of electric components, such as printed circuit boards and resistors. The identities of these gases are not described. It merely describes how an ion mobility spectrum changes when gases are released from heated lacquer coated printed circuit boards and supplied to the spectrometer. A substantial disadvantage of this method is that it requires significant down time during saturation of the measured values by other gases which are released by a formation of a fire. This means that for a major time period surveillance with regard to the formation of a fire does not occur.
The main principle of ion mobility spectrometry is based on the fact that ions formed under normal pressure, drift against the flow direction of a gas in an electrical field. Ions of different mass and/or structure achieve different drift velocities and are separated until they impinge a detector chronologically one after another. The ratio between the ion drift velocity and the force of the electric field is referred to as ion mobility and the separation of these ions along a determined distance based on the different drifting velocities is referred to as ion mobility spectrometry. The low field strengths and the resulting independence of the ion mobility from the field are characteristics of this method.
An ion mobility spectrometer consists substantially of a drift tube which in turn consists of a reaction chamber and a drift chamber. Both chambers are separated from each other by an electric switching grid.
A disadvantage of this method is that electric switching grids are required for admission of samples and that shielding grids are required in front of the detector. Thus, the detector has greater dimensions and is often more expensive. It can furthermore be disadvantageous that either only positive or only negative ions are measured, while a detection of both positive and negative ions as fire formation parameters is not possible.
US Patent Publication No. 2008/0128609A1 describes a method and device which is a further development of an ion mobility spectrometer. It concerns a system for differential ion mobility spectrometry (DMS). Such DMS systems use the field dependence of the ion mobility in order to obtain a higher selectivity in substance identification. Samples to be analyzed are extensively prepared, ionized and the ions are channeled by a high-frequency alternating electric field by means of a compensation voltage to measurement electrodes. Magnetic fields for influencing the ion trajectories are not used.
The example 10 of US Patent Publication No. 2008/0128609A1 and the publication “GC-PFAIMS as Smart Smoke Alarm, G. A. Eiceman et al, Vol. 5, No. 3, 2002, pages 71-75, XP-002544793” describes the chemical analysis of volatile decomposition products of cotton, paper, grass and motor exhaust gases by means of the DMS method and device, which pre-connects a gas chromatograph (GC) to the DMS System for pre-separation. The method and the analysis are divided into five substantial steps. First, the ignition of the sample, secondly, the enrichment of the forming decomposition gases on a SPME-fiber (Solid Phase Micro-Extraction), thirdly, the thermal desorption of the enriched gases of this fiber, fourthly, the chromatographic separation of the gases and fifthly, the substance identification with the downstream DMS System. In addition, nitrogen is hereby used as a carrier gas for transporting the substances to be analyzed through the electric field. This is another additional undesirable technical expense.
The presented solution does not constitute a fire detection system but merely shows the potential possibilities allowing substance identification and a differentiation of smoke gases from different sources with the described GC/DMS-based highly complex chemical analysis.
Disadvantages of this solution include highly complex equipment (and thus high costs) caused by pre-separating the substances with a GC, the sample preparation as well as a long duration of the analysis. Another disadvantage of the spectrometric GC/DMS method for fire detection is the need to compare the measured values with known identification data in order to allow a classification and substance identification. With regard to fire detection, this means that all of the substances that are to be identified must be previously known and that this identification data must be stored in the memory of the spectrometer.
A fire gas or smoke detector is known from the document DE100357371A1 which uses an ion current, which is related to the appearance of fire gases or fire smoke, and which can be measured in case of a high voltage between a corona-discharge electrode (cathode) and a suction electrode (anode). The disadvantage of this method is that all of the supplied substances contained in the air and not only the products of a thermal decomposition from a forming fire, which are ionized by the corona discharge, contribute to the fire alarm. One must thus expect a high rate of false alarms (falsely positive detection).